FIG. 20 is illustrative in section and schematic of one example of prior art imaging optical systems. The imaging optical system comprises, in order from its object side (the left side of FIG. 20), a first lens group G1 having a positive focal length and a second lens group G2 having a negative focal length with a stop S interposed between them. The first lens group G1 comprises, in order from its object side, a positive first lens L1, a negative second lens group L2, and a positive third lens L3 and a fourth lens L4 having a cementing surface, and the second lens group G2 comprises a fifth lens L5 defined by a positive meniscus lens convex on an imaging plane side and a sixth lens L6 defined by a negative meniscus lens concave on the object side. Light emanating out of a subject passes through the first lens L1 to the sixth lens L6, and is imaged on an imaging plane 6 with a CCD image sensor placed on it, where the image of the subject is taken.
Well now, current trends in recent imaging optical systems are to reduce their size, weight and cost, and with this, molded optical elements more improved in terms of mass productivity and cost than ordinary polished optical elements are more frequently used for optical elements used with them, too. Molded optical elements are preferably fabricated by molding glasses or resins using molds.
In cutting of the mold used for such molding, there is the need of simplifying the process involved and keeping processing precision high; to this end, cutting is generally implemented by operating a cutting tool at equal pitch in a certain direction or operating a cutting tool at equal speed on a constantly rotating mold member.
However, when the mold is prepared by operating a cutting tool at equal pitch in a certain direction or operating a cutting tool at equal speed on a constantly rotating mold member, there is a minute, periodic streak produced depending on the feed pitch or the rotation speed and feed speed.
As the mold having such a streak is used for glass or resin molding, it is transferred, yielding a molded optical element having a periodic streak, too.
As high-brightness subject light inclusive of sunlight, light from light sources, illumination light and light from emitters is taken with a digital camera's imaging apparatus comprising a molded optical element having such a streak, there is diffracted light generated from that streak on the surface of the molded optical element. As shown in FIG. 22, this diffracted light is concurrently incident on a position different from the position with a light source image 10 formed at it, forming a nonessential light image 11. Note here that the optical system of FIG. 20 is used as the imaging optical system 7 of FIG. 22, and the 7th optical surface (the surface of the 4th lens L4 on the imaging plane side) r7 is supposed to have a periodic streak 2.
A problem with the periodic streak 2 thus produced on the molded optical element surface is that, as shown in FIG. 22, nonessential light occurs, giving rise to image quality deterioration.
To reduce or eliminate such a nonessential light image, the method for removing that streak by a polishing step has been used so far in the art. With that method, however, there is a risk of design surface shape breaking down, resulting in a lowering of resolving power.
It is noted that when the optical element having such a periodic streak is used with an image reader using a line sensor and a rectangular optical element, it has been proposed in Patent Publication 1 to get rid of influences of diffracted light from that steak. However, this is not applicable to an image apparatus using two-dimensional light receptor elements as herein contemplated.
[Patent Publication 1]
JP(A)2005-331715